Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Earnings Per Share [Abstract]
Income from continuing operations									$ 50,041	$ 54,831	$ 49,159
Income from discontinued operations, net of income attributable to the noncontrolling interest									624,346	24,592	17,270
Net income attributable to Alexander's	$ 617,157	$ 18,856	$ 18,892	$ 19,482	$ 20,634	$ 20,425	$ 20,157	$ 18,207	$ 674,387	$ 79,423	$ 66,429
Weighted average shares outstanding - basic and diluted (in shares)									5,107,610	5,106,568	5,105,936
Income from continuing operations- basic and diluted (in dollars per share)									$ 9.80	$ 10.74	$ 9.63
Income from discontinued operations, net - basic and diluted (in dollars per share)									$ 122.24	$ 4.81	$ 3.38
Net income per common share (in dollars per share)									$ 132.04	$ 15.55	$ 13.01